Oxidation of fatty acids



Dec. 4, 1956 R. D. ENGLERT ETAI-l OXIDATION oF FATTI ACIDS Filed Dec. 14. 1953 United St y zg-'lflsgoslt JOXIDTI'ON OFFATIY ACIDS Robert D. EnglertJalo Alto, and Lawrence M.` Richards, LosV -Altos, Calif., assignorso Tallow'Researcl'l, Inc.,

jSanfFrancisco, Calif., acorporation ofClifornia Application December 14, 1953;'SerialNo. 398020 .lmClaims (Cl. 2'60-'537) This invention relates to a process of making organic acids/from.fatty;acids, particularly .fatty acids of 14 to jllcarbonato'mst More particularly it relates to the oxi- VArlati'onof fatty acidswithl nitric acid toproduce mono- 'fbasicV andV dibasic acids of 1ower molecular weight. This i applicationgis a continuationinjpartj'of our-.application :Ser. No.3:l'8,252, tiled-November .1, w19-5v2. j v One object of theiinventionisfto e'iec-t oxidationl an splitting .of saturatediattyV acids Usuch* asf palmitic and .and air,.usually witha. catalyst. Because yof the stability of the fatty ac'ids,severe oxidizing conditions ,have been r,.required. Eonexample, it -hasvbeen the practice .to fuse c ofncentrated,,or fumingnitric acid, resultng'inhigh acid .consumption and Iproduction of. CO2 and NO gases. .In order .t-o facilitatexthe oxidation, ithas beenthe-,practice ...to .employ unsaturatedfatty acids or .hydroxy acids .such as .hydroxy .stearic .or ricinoleic acids which .are .relatively .easilyattacked .bytheoxidizingagent.. Ithas not been considered feasible to -oxidize saturated. fatty acids bek ctnise ,of theirdiighr resistanceto oxidation, ,even when .employingpowerfuloxidizing agentssuch as chrornic .or

concentrated nitric acids. In the p resence of `thesefstrong 4 oxidizing agents, the initialproducts of the reactionhave @,b, en oxidized'further,tosimple compounds Oflittle value s uch. as acetic acid or almost no value such as C02 and CO. K

'. We havenow discovered rthat saturated A.fatty acids, t'particular1y palmitic and stearic acids, can be oxidized t yaluableacids of lowermolecular weight, both monofbasio and .di-.basic by the action of fdiluted .nitric `acid a Levtedtemperatureand pressure. We .prefer to ,use 'acidof about 10 to 30% concentration,'1520%. -being verysatisfactory, and higher concentrations up to 50% can.,l3e .,en1ployed Withlclose.temperature control. The

tes Patent densethewater and acids so lthat, only the fixed Vgases reaction is rapid'with go'odragitation to eiect'contact between the fatty acid'and aqueous nitric acid phases, and

-nor catalyst is required.v

Y desirable toc'perateat a temperature approximat- Ling the boiling point ofthe nitric acid employedLnnder" "720% HNO3*a -temperature of n170uv C. was' satisfactory.

In genera1,.the temperature should be betweenabout 150,0 Y and 250 C. depending -on `the -acid concentration and Y dilute acid andZor when convertingflargelyrto dibasicyacids.

The reaction can beconductediat `the vaporpressure [sof the ,reaction mixture, and we may also ernployagreater .pressureby introducing a gas suchI as air, nitrogen, VNO2V v o1-.carbon:.dioxidej. Pressures Iin V.tlleratlgezof .about 1.0()

2,773,094 Patented Dec. 4, 1.1956

ICC

vtOfZOOjQp. s. jg; areisuicient`for"rnostfpurposes,'and ,we

"When conducting thereaction in "anautoc`lave provided with an agitator, the fatty acidand nitricacidarecharged then`the"vessel'i;is closedxand the temperature raised to "the"desiredpjpoint. i Thelpr'essure developed from the expandingvaporsof waterand acids'together-with theggases kevolvedfin the reaction', such asifCOz, N2 andNO,` may "be 'held atany desired,point byiventing "thru a. pressure controlvalve. 'Internally coolingthe vapors servesito con- Vare fre-moved from the autoclave. Fixedil gases can also bejcharged to the ...autoclave at lthe starty before heating -toprovidean initial pressure and assist in .controlling the reaction. l v

vReferring to fthe drawing, 'fatty acid, e. g. stearic, palmitic, oleic acid, or mixed acids from animal-vege- Jtable or'rnarine fats, isi charged byline '10 to reaction `@vessel 11' where it is .brought into intimate contact 'with nitric acid introduced 'byline 12.` Efficient agitationlis `provided'by agitator 13 which can beamultibladedpropeller orother suitable .type 4of agitator effective ,in dispersing the .nitric acid'. and 4the fatty acid, ,producing extensive surface `and high reaction rate. Coil 14 serves Lto'controlthe'ternperature by'heatingorcooling asneeded, suitably..by'heatirigwith steam or `hot water, or cooling wit-h'water, cold; brine or other refrigerant. VAfter the reaction'has been brought to .the desired temperature, `e. g. Z50-300 F., it is'"ge'nerll`y necessary :to supply .refrigeration to .carry away Athe exotherinic lheat of oxidati'on.

i i The'time of residencewf'fatty acidinreactor v 11 islpreferably about .ten seconds tofiive minutes .depending Von the rate'of agitation, temperature .and :acid concentration each of.k which speeds the oxidation when increased.

vOwing tokthe high agitation and extensive surface between the aqueous Aphase land the fatty .acid phase, the latter 'is' largely oxidized tol 1ower molecular weight Vorganic .acidsby aprocess of chain `splittingresulting in `the .format'ion of monobasic andl dibasic .acids `as .theprincipal Vreaction product. nitrogen, carbon monoxide, carbon dioxide, and nitric `0 xide`,"l'\l0. The entire-reaction .product is continuously removedfby'linevlS leadingl to .stripper 16 where the gases and the more volatile monobasicv acids are removed vby `vapor line v17 along with steam and nitricacid.`

We'liave .found it ,advantageous to .use `nitric acid vof about 10-30'% concentrationw-in waterv at a pressure. of about 3 to 1,0 atmospheres. .Somewhat 'higher concentrations of acid vcan Ybe. used, e. g. l4G-.509@ but: offer no v`ladvantage -in yield or conversion, and the'useof :more concentratedracid` increases the fcost ofthe operation, particularly .in'acid recovery `and acid losses. Use .of more concentrated acid, above. about '30%, increases the corrosionrproblem with materials' for apparatus: construction. :The mol ratio of nitric acid to fatty acid chargedV l iis Vusually in- ,the range '.10 or 1210.1. N

.off4z1 to 3021,:.preferably about The pressure i'sicontrolled by valve 18 which :can-be 4adjusted to hold any pressuredesired -on reactor 11. We may also extendlthereactionntimefby'diverting the productsfrom reactorl 1^1by-valved :line -19 to separator 20 iii/herethezadmixedgaseous Ilay-products are withdrawn withilsom'e lvvapors ofnitric` acid, water and monobasic acids,.lthe vapors,V passing by linel21 to coo1er 22, thence to receiver 23- whereco'ndensate is collected and cool gases are discharged bylinef24. Pressure is preferably reduced-byvalve-ZS from-fthe reactionv pressure to substantially atmospheric, e. g. 5 to 1 0,p...s. img..

tator, or fthe stream of reaction productscan-.bernader'o pass over .baiiesor Vthru.apaclcingeg. stoneware Vsadtglles Gaseousb .y;products` of 1 the `reaction are Y Separator 20 can be providedv with;'amechanicalagir.`

or rings, to prevent separation of phases and extend the time of reaction of the fatty acid by the hot nitric acid for example from 5 to 30 minutes. From 20, the products ow thru valve 26 and line 15 to stripper 16, valve 27 being closed when operating with separator 20. The volume of separator may equal or exceed the volume of reactor 11 by 2 to 2O fold.

' Steam can be introduced by line 28 to the base of stripper 16 to remove more of the volatile acids from the reaction products, mainly monobasic acids ranging from acetic to pelargonic acid or higher. Valerio, caproic, oenanthylic and caprylic acids are also formed from saturated fatty acids and steam distilled from'the dibasic acids in stripper 16. The vapors in line 17 are condensed in condenser 29 and the condensate llows to receiver 23 where uncondensed gases pass out at 24. The water soluble acids form a layer which collects on the surface. Acids from this layer are drawn olf continuously or intermittently at 31 and may be redistilled by means not shown.

The soluble, volatile acids, principally propionic and butyric acids, withdrawn by line are recovered for example by extraction, from the dilute nitric acid in which they areV dissolved, and the latter is recycled to the reaction chamber 11 by means not shown.

Returning now to stripper 16, the stripped products from the bottom pass by line 32 to trap 33`where unreacted fatty acids and other insoluble, relatively nonvolatile acids are withdrawn as an upper layer at line 34. 'These acids can be recycled to the reactor 11 directly or after redistillation to recover higher boiling monobasic acids of lower molecularV weight than the fatty acids charged to the process. i

From the bottom of trap 33 the dibasic acids pass by` line 35 to extractor 36. Alternatively they can be chilled in cooler 37 to crystallize part of the disbasic acids which are separated by iilter 38,whence the filtrate is passed to extractor 36 by line 35.

In extractor 36, the descending stream of dibasic organic acids contact a selective solvent which removes them fom the dilute nitric acid, the latter passing by line 39 through heater 40 to stripper 41 where solvent vapors are removed, led by line 42 to condenser 43 and returned to solvent storage tank 44.V The solvent is then returned to extractor 36' by pump 45 and line 46.

From the base of stripper 41, dilute nitric acid canV be withdrawn from the system by, line 47 or recycled to reactor 11 by line 48 and pump 49. Before-entering reactor 11, the Vrecycled nitric acid can be fortified by introduction of strong nitric acid from line12, e. g. 70% -HNOs can be employed for Vmake up acid to regulate the concentration in reactor 11. Dibasic acids in solution are conducted by line 50 thru heater v51 to evaporator 52 where solvent is recovered from the acids which are crystallized and discharged at 53.' Solvent vapors pass by line 54 to'condenser S5, thence by line 56 to solvent supply 44. Vacuum evaporation may be used if desired. Y

One of the better solvents for extracting dibasic acids 'from dilute nitric acid is methyl isobutyl ketone-MBK.

The following table shows the Vsolubilities of two samples' of mixed dibasic acids in this solvent, in water and in nitric acid:

solubility of dibasic acids in MIRK- Azelaic, 9 cai-bom` Solubility of azelac acid Ratio of Solvent Solubility, Solubilit gm./100 cc. Solvent 20% HNO:

2-E thyl hexanol-l. 8. 9 7. 4 Di-isobutyl carbino 8. 6 7. 2 Trlmethyl nonanol- 7. 2 6. 0 Ethyl Acetate 4. 9 4. 1 Methyl isobutyl ketone. 4. 2 3. 5 Methyl n-amyl ketone. 3. 9 3. 3 Ethyl Butyl Ketone 3. 3 2. 8 Capl'ylic-Caprlc Acid Mixture 5.0 4. 2 Crude monobasic acid traction from oleic acid oxidation (nent. eq.= pelargonic acid) 6. 2 5. 2 20% nitric acid 1. 2

' boiling Aalcohols can be used satisfactorily if the dissolved acids are extracted from them with a suitable base, such as-sodium hydroxide, ammonium hydroxide, etc.

-We canuse monobasic acids produced in the process to extract thedibasic acid product and for this purpose we prefer to use a fraction of selected volatility to aid Vseparation from the relatively nonvolatile dibasic acids.

Thus a Water insoluble Afraction of Ymixed monobasic acids having about six to nine carbon atoms is quite suitable. When using these monobasicacids as solvents, it is not necessaryto recover them from the nitric acid `which* is recycled to the reactorll, inasmuch as the monobasic acids recycled to the reactor to valuable dibasic acids. l

Following is the composition of a'mixture ofrdibasic acids which we obtained fromY the oxidation of palmitic acid in an autoclave. Six-tenths mol of palmitic acid pure) was heated and agitated 30 minutes with 2266 cc. of nitric acid-20%, at 170 C. The pressure are converted therein "was adjusted with nitrogen to p. s. i. g. before heating, andv held there during the reaction.v Fifty percent of the palmitic acid was converted, the yield of dibasic acids (calculated as azelaic) being 99% of the Vacid converted.

Composition of dibasicV acids from palmitick VacidA Comp., Mol Y Percent Acid Suceinic, 4 carbons Glutaric, 5 carbone Adipic, 6 carbons. Pimelic, 7 carbons Suberic, 8 carbons Sebacic, 10 carbone v Undeeancdioic, 11'earhons Dodecanedioic, 12 carbone- Tridecanedioic, 13 carbone. Tetradeeanedioic, 14 fcarbons..-

' 'Theunconverted paliniticY acid from the jabove was found towcontain 5.4% nitrogen which was found to. be mostly in nitrcso derivatives, about 10% being 1n nitro Y unconverted @fatty acid,

derivatives, apparently intermediate reaction products. A portion of the mixed dibasic acids was esteriiied with butyl alcohol, giving the dibutyl ester, a valuable plasticizer for plastics, and resins. p

Another autoclave oxidation of palmitic acid was run as before but the charge (153.6 gms.) was pressured with CO2, to 50 p. s. i. g. before heating. Analysis ofthe products was made in the followingmanner: one-half was worked up for monobasic and dibasic acids and unreacted palrnitic acid. The other half was used to determine nitric acid balance. Nitrogen in the organic material was determined by the Dumas method. Unused HNO; was determined by titration, correcting for extractiole organic acids. The results are given in the following table:

Products from palmitic acid oxidation Product Weight Percent (Grams) Unreacted palmitic acid Monobasic acids (calculated as R-CHz) Dlbasic Acids (Calculated as CHz-R-COZH) CO2 and CO (Calculated as CH2) Not accounted for;

Nitric acid balance Combined Organic Of the nitric acid charged, 97% was accounted for.

i The foregoing dataY are typical of the results obtained V1n batch operation with saturated fatty acids such as myristic, palmitic, stearic and arachidic. The product acids are readily separated from each other by distillation and/or crystallization in a vmanner well known inV the art. For use as plasticizers, the dibasic acids can Y be converted to their esters, for example the dioctylor dinonyl esters. These esters are also valuable lubricating oils for use 1n critical service in aviation because of their Vexcellent low temperature and high temperature charac- Vteristics. For this purpose, mixtures can be used and are generally preferred.

Altho our process is directed to the oxidation of saturated fatty acids, it is not necessary to use pure, saturated acidsas charging stock butnlixed acids containing substantial` amounts of unsaturated acids can be used and We prefer to use the natural fatty acids derived from beef tallow, sperm oil, coconut oil, horse fat, mutton tallow, palm nutv oil, peanut oil, tall oil, and Wool fat. VThe acids derived from these fats, for example by use of the vTwitchell acid saponication, generally have iodine values less than about 100 andV usually in the range of about 25 to 75. Monobasic acids of higher molecular weight, e. g. l0 to 14 carbon atoms, are usually allowed to remain with the-unconverted fatty acid which isY recycled to the oxidation reaction.

VvHaving thus describedV our invention what We claim is: 1. The process of converting saturated fatty acids of Y. 14 to 18 carbon atomsto Vdibasic acids of lower molecularrweight which comprises intimately contacting said fattyacids withA an oxidizing agent consisting of dilute nitric acid at a temperature of 100 to 250 C. and a pressure in the range-of about 50 to 500 pjs. i. gage, the mol j `ratio of nitric acid tofattyY acid charged being within the range of about 4:,1 to 30:1 and separating from the molecular weight.

dibasic organic-acids of. lower 75 2. The proces of claim 1 wherein the nitric acid concentration is about 10 to 30% HNOs.

3. The process of claim 1 wherein volatile monobasic acids are separated from the reaction products by distillation and the nitric acid is recycled to the conversion step of the process. Y

4.*The processor' converting saturated fatty, acids of 14 to 18 carbon atoms into dibasic organic acids of lower molecular Weight which comprises continuously charging said fatty acid and an oxidizing agent consisting of nitric acid to a reaction zone, maintaining the concentration of nitric acid in said reaction zone at about 5 to 30 percent with respect to the water present, maintaining the mol ratio of nitric acid to said fatty acid charged within the range of about 4:1 to 30:1, vigorously agitating the contents of said reaction zone to provide intimate contact between the three phases therein, oil, aqueous and gaseous, maintaining a pressure on said reaction zone between about 50 and 500 p. s. 1. gage, controlling the temperature of said reaction zone within the range of about -250 C., continuously withdrawing reaction product from said reaction zone, reducing the pressure in a distillation zone and distilling volatile organic `acids therefrom, and thereafter separating water soluble, relatively nonvolatile dibasic acids from unreacted fatty acids.

5. The process of claim 4 wherein said unreacted fatty acids are recycled to said reaction zone.

6. The process of claim 4 wherein nitric acid is recovered from said dibasic acids by extraction with a selective solvent and recycled to said reaction zone.

7. The process of claim 4 wherein the temperature of siad reaction Zone is about to 200 C.

8. The process of claim 5 wherein the lower molecular weight monobasic acids remaining with said unreacted fatty acids are distilled therefrom before said fatty acids are recycled to said reaction zone. i

9. The process of converting fatty acids of 14 to 18 carbon atoms to dibasic acidswith a lesser number of Vcarbon atoms which comprises continuously chargingV said fatty acid toa reaction zone and agitating it thereinV with an oxidizing` agent consisting of nitric acid of about ten to fty percent concentration, maintaining the mol ratio of nitric acid to fatty acid charged to said reactionY zone within the range of about 4:1 to 30:1, maintaining the temperature of said reaction zone at a point within the range of about to 250 C., maintaining the pressure' of said reaction zone at least at the vapor pressure of the nitric acid therein, continuously removing reaction products from said reaction zone without separation ,and expanding them into a vaporizing'zone at` lower pressure, recovering volatile'pro'ducts from said vaporizing zone and recovering dibasic acids from the non-volatile yfraction withdrawn from said vaporizing zone.

10. 'Ihe process of claim 9 wherein said fatty acids to be converted have an iodine value in the range of 25 to 75.

11. The process of claim 9 wherein the non-volatile products from said vaporizing zone are separated into an insoluble layer comprised of unreacted fatty acids and an aqueous layer containing dibasic acids, and the unreacted fatty acids are recycled to the reaction zone.

12. The process of claim 11 wherein the dibasic acids are recovered fromthe aqueous layer by solvent extraction.V

13. The process of claim 112V wherein the solvent used inV said extraction is selected Yfrom theclass consistingY of methyl iSOblltylv ketone, methyl n-amyl ketone` and ethyl butyl ketone. 1,. v y

References Cited in the le of this patent` UNITED STATES PATENTS 2,662,908 Logn1 13m15.195s` 

1. THE PROCESS OF CONVERTING SATURATED FATTY ACIDS OF 14 TO 18 CARBON ATOMS TO DIBASIC ACIDS OF LOWER MOLECULAR WEIGHT WHICH COMPRISES INTIMATELY CONTACTING SAID FATTY ACIDS WITH AN OXIDIZING AGENT CONSISTING OF DILUTE NITRIC ACID AT A TEMPERATURE OF 100 TO 250* C. AND A PRESSURE IN THE RANGE OF ABOUT 50 TO 500 P. S. I. GAGE, THE MOL RATIO OF NITRIC ACID TO FATTY ACID CHARGED BEING WITHIN THE RANGE OF ABOUT 4:1 TO 30:1 AND SEPARATING FROM THE UNCOVERTED FATTY ACID, DIBASIC ORGANIC ACIDS OF LOWER MOLECULAR WEIGHT. 